Implantable medical systems that provide electrical stimulation therapy and/or sense physiological parameters of a patient typically include an implantable medical device (IMD) coupled to one or more electrical stimulation leads. IMDs may deliver therapy to or monitor conditions of a variety of organs, nerves, muscle or tissue, such as the heart, brain, stomach, spinal cord, pelvic floor, and the like. In the case of implantable cardiac systems, the IMD may be configured to provide electrical stimulation therapy to the heart and/or sense electrical activity of the heart via the electrical stimulation lead(s). One such IMD, the implantable cardioverter-defibrillator (ICD), may be capable of delivering both low voltage therapies and high voltage therapies in response to monitoring a cardiac rhythm and detecting a need for therapy.
Low voltage therapies may include bradycardia pacing, cardiac resynchronization therapy (CRT), and anti-tachycardia pacing (ATP). Low voltage therapies are typically delivered using low voltage pacing electrodes, e.g., tip or ring electrodes, together or in conjunction with the housing of the ICD, often referred to as the “CAN electrode” or a “housing electrode.” Low voltage therapies may include pulses delivered at 5 volts (V) or less in amplitude. High voltage therapies, such as cardioversion or defibrillation shocks, are delivered in response to detecting ventricular tachycardia (VT) or ventricular fibrillation (VF). High voltage therapies are typically delivered using high voltage coil electrodes and the housing or CAN electrode. High voltage electrodes generally have a greater surface area than low voltage electrodes and deliver high energy shock pulses, typically in the range of at least 10 Joules and up to 35 Joules for transvenous systems and in the range of at least 65 Joules and up to 80 Joules for subcutaneous systems.
A single lead coupled to an ICD may carry multiple electrodes, which may include either or both high voltage and low voltage electrodes. Each electrode is electrically coupled to an electrically insulated conductor extending through the elongated lead body to facilitate electrical connection of each electrode to circuitry within the ICD. Lead problems can sometimes occur when an electrode, or the conductor to which it is electrically coupled, makes electrical contact with another conductor or electrode including the housing electrode. These lead problems may result in inappropriate sensing (e.g. oversensing) or in inadequate delivery of stimulation therapy.